1. Field of the Invention
The present invention relates to a method of measuring a physical variation in a static structure (a bridge, a building and so on) or a dynamic structure (an airplane, a vehicle and so on) using an optical fiber and Brillouin scattering of the optical fiber. More specifically, the invention relates to a physical quantity measuring method using a Brillouin optical fiber sensor, which is able to measure a physical quantity and its distribution of a corresponding structure with improved spatial resolution using two pulse lights and two back scattering lights acquired from the pulse lights.
2. Background of the Related Art
It is generally known that Brillouin scattering relates to a sound wave generated according to movement of molecules physically excited in a gas, liquid or solid. There has been developed a Brillouin optical fiber sensor using the Brillouin scattering of an optical fiber. The Brillouin sensor has a structure that a pulse light is reacted with an oscillating wave inside an optical fiber when the pulse light is transmitted through the optical fiber such that the pulse light is scattered at a frequency different from the frequency of the original pulse light. If a specific physical quantity (heat, load and so on) is applied to the optical fiber, molecules of the optical fiber, which are excited due to the physical quantity, form a sound wave and act on the pulse light. Accordingly, the scattered pulse light has a frequency different from the frequency of the pulse light when the pulse light is input to the optical fiber. The physical quantity can be measured by detecting the frequency of the scattered pulse light. Thus, the Brillouin optical fiber sensor can be utilized as a sensing structure.
FIG. 1 shows the construction of a conventional Brillouin sensor and FIG. 2 shows a Brillouin gain spectrum obtained by measuring temperature distribution of a specific structure using the sensor of FIG. 1.
Referring to FIG. 1, first and second light sources 20 and 30 are respectively arranged at both ends of an optical fiber 10, opposite to each other. The first light source 20 arranged at one end of the optical fiber 10 transmits a pumping pulse light to the other end of the optical fiber 10. The second light source 30 located at the other end of the optical fiber 10 transmits a continuous wave probe light to one end of the optical fiber 10. A portion of the optical fiber 10, exposed between the first and second light sources 20 and 30, is attached to a specific structure (a building, a bridge and so on, for instance) to serve as a sensing part.
When a light receiver 40 is connected to the optical fiber 10, the portion of the optical fiber 10, which is attached to the structure, functions as a sensing structure. Thus, the frequency of scattered light transmitted in response to a variation in the temperature of the structure can be compared with a difference between the frequency of the pumping pulse light and the frequency of the probe light. Furthermore, a structure capable of measuring a temperature is based on the fact that the frequency of the scattered light is changed as a temperature becomes changed.
When the frequency of the pumping pulse light input to one end of the optical fiber 10 is vp and the frequency of the continuous wave probe light input to the other end of the optical fiber 10 is vcw, the difference between the frequencies of the pumping pulse light and probe light corresponds to Δv=vp−vcw.
When the frequencies of the pumping pulse light and probe light are adjusted such that Δv corresponds to a Brillouin frequency shift of the optical fiber 10, the pumping pulse light is photo-energy-converted into the probe light and thus the probe light is amplified as Brillouin-light inside the optical fiber 10. Accordingly, analysis of Brillouin signals is facilitated.
The amplified probe light signal is converted into an electric signal by the light receiver 40. The electric signal has Brillouin gain spectrum characteristic shown in FIG. 2 based on Δv and time detected by the light receiver.
When the optical fiber 10 attached to the surface of the structure is subjected to a variation due to a temperature, for example, the Brillouin frequency value of the optical fiber 10 is changed as shown in FIG. 2.
The molecules of the optical fiber are thermally excited when the probe light meets the pumping pulse light to cause sound oscillation and generate Brillouin scattering light. This Brillouin scattering amplification is generated when the difference between the frequencies of the pumping pulse light and probe light has a specific value, which is the Brillouin frequency value. The Brillouin frequency value is a property of the optical fiber and it is varied in proportion to a physical quantity such as a temperature or strain.
When the Brillouin frequency shift corresponds to Δv, the maximum power of the optical fiber is obtained. FIG. 2 shows the distribution of the frequency shift value based on a frequency axis, an optical intensity axis (mW) and a length (km) of the sensing part of the optical fiber 10. The frequency shift Δv is shown to 10.8 GHz, 10.85 GHz, 10.9 GHz and 10.95 GHz on the frequency axis. A temperature variation based on the distribution of the frequency shift is shown as a graph above the graph showing the distribution of the frequency shift. The larger the number of Δv frequency shifts set on the frequency axis, the more accurate the distribution of the frequency shift is.
However, the conventional Brillouin sensor requires the structure in which the two light sources, that is, the first and second light sources 20 and 30, are arranged at both ends of the optical fiber 10 in order to acquire data about Δv and needs a very long period of time to obtain measurement results because the probe light of the second light source 30 is emitted in the form of pulse light. Accordingly, when the conventional Brillouin sensor is applied to a large structure, a factor causing a temperature variation, for example, a fire, cannot be immediately detected.
In the Brillouin optical fiber sensor structure, it is very important how long portion of the optical fiber is effective as a sensing part used for measuring a physical quantity. The size (or length) of a portion of the Brillouin optical fiber sensor, affected by a physical quantity based on a measured value at an arbitrary position, is called spatial resolution.
When the spatial resolution of an optical fiber sensor is 1 m, for example, it means that a single sensing signal is obtained within 1 m of a corresponding portion of the optical fiber. It is known that the spatial resolution is directly proportional to the width of the pumping pulse light in the optical fiber sensor structure.
A portion of the optical fiber, in which pumping pulse light is back-scattered, corresponds to half of the length of the optical fiber, occupied by the pumping pulse light. Thus, when the width of the pulse light is 10 nsec, the spatial resolution becomes 1 m (that is equal to 200,000 km/s×10 nsec×½) considering that the velocity of light transmitted inside the optical fiber is approximately 200,000 km/s.
Accordingly, a signal having the spatial resolution of 1 m is obtained when the optical fiber sensor is operated using the pumping pulse light with the pulse width of 10 nsec. To obtain the more accurate signal from the corresponding length of the optical fiber, it is required to operate the optical fiber sensor with smaller spatial resolution. In this case, however, the scattering light is decreased because the energy of the pumping pulse light is reduced.
It is known that the Brillouin scattering light amplification occurs only when a pulse light with a pulse width of at least 10 nsec is used and it is difficult to determine a gain peak with a pulse width of less than 50 nsec because the line width of the Brillouin scattering spectrum is remarkably increased.